1. Field of the Invention
This invention relates to the technical field of automatic transmissions with stepped gears comprising a hydraulic torque converter able to bridge on certain gears.
2. Discussion of Background
It has become common practice to equip automatic transmissions with a bridging system--or lock-up--in the converter, to make the turbine and the impeller integral under certain engine speed and load conditions, which makes it possible to eliminate the considerable slipping of the converter and thus to improve the efficiency of the transmission under the conditions in question. Control of the bridging is performed most often by on or off, which implies that during the phases where the lock-up is closed, a damper is provided to best minimize the effect of the acyclic motions of the engine throughout the kinematic chain.
On this subject, publication EP 179 683 of the applicant describes a stepped automatic transmission control device having four main forward gears and one reverse gear, equipped with a hydrokinetic torque converter equipped with a torsion vibration damper intended to filter the irregularities of the engine torque on the gears for which the converter does not intervene in the transmission of the engine torque.
To eliminate the torsion damper which is difficult to house in the shell of the converter and, based on an often limited and inadequate efficiency, it has been proposed to make, in a controlled and very limited manner, the bridging clutch slip so as to make it play a role of instantaneous torque limiter, which then imparts to it a capability of filtering torsion vibrations produced by the acyclic motions of the engine. Nevertheless, this idea, perfectly correct in its principle, comes up against significant difficulties of practical use, because it is extremely difficult, indeed impossible, to find in the very constrained dimensions of a converter the necessary space to install a clutch having a friction surface that is suitable and sufficiently slightly charged with pressure to guarantee an error-free operation for the entire life of the vehicle. Moreover, the considerable deformations that a converter undergoes during its operation further complicate the problem, because it is important then to find mountings making it possible to separate the friction surfaces of the actual elements of the converter to guarantee, in any circumstances, a surface evenness or a perfect coupling of the friction surfaces in contact which, alone, make it possible to expect a sufficient operating life.
Under these conditions, the design of a bridging converter with controlled slipping constitutes an arduous task, whenever the space available axially for the converter is limited, which is the case in most of the transverse structures, for traction vehicles.